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AN897 Datasheet(PDF) 8 Page - Microchip Technology

Part # AN897
Description  Thermistor Temperature Sensing with MCP6SX2 PGAs
PDF  16 Pages
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Manufacturer  MICROCHIP [Microchip Technology]
Direct Link  http://www.microchip.com
Logo MICROCHIP - Microchip Technology

AN897 Datasheet(HTML) 8 Page - Microchip Technology

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AN897
DS00897B-page 8
 2004 Microchip Technology Inc.
FIGURE 19:
ADC Temperature
Resolution.
Figure 20 compares the analog circuit errors between
the designs. The second design’s error is better at high
temperatures because the ADC’s temperature resolu-
tion is better. It is also better at low temperatures
because RA has been selected to linearize the
temperature-to-voltage conversion there.
FIGURE 20:
Analog Circuit Error
Comparison.
The digital piece-wise linear interpolation errors are
compared in Figure 21. The second design has much
better performance because the linear interpolation
table segments cover smaller changes in temperature.
FIGURE 21:
Digital Interpolation Error
Comparison.
Figure 22 compares the total errors (thermistor plus
circuit plus piece-wise linear interpolation) of the first
and second designs. The digital roundoff error has
been excluded for simplicity.
FIGURE 22:
Total Error Comparison.
Other
trade-offs
between
the
two
designs
are
summarized in Table 4.
TABLE 4:
TRADE-OFFS.
MEASURED RESULTS
Both designs were measured on the bench. The
thermistor was emulated with the variable resistor Rvar
on the board shown in Appendix A.1 “Thermistor
PGA PICtail Daughter Board”. The ADC outputs were
converted to estimated thermistor temperatures using
nominal resistor values. Figure 23 shows the first
design’s measured error, while Figure 24 shows the
second design’s measured error.
FIGURE 23:
Measured Errors,
Design # 1.
-2.5
-2.0
-1.5
-1.0
-0.5
0.0
-50
-25
0
25
50
75
100
125
150
Thermistor Temperature (°C)
2
nd Design
1
st Design
10-bit ADC
DC Error
≤ 3.5 LSb
0
1
2
3
4
5
6
7
8
9
10
11
-50
-25
0
25
50
75
100
125
150
Thermistor Temperature (°C)
Design # 1
Design # 2
-1.0
-0.8
-0.6
-0.4
-0.2
0.0
0.2
0.4
0.6
0.8
1.0
-50
-25
0
25
50
75
100
125
150
Thermistor Temperature (°C)
Design # 1
Design # 2
Criteria
First
Design
Second
Design
Temperature Range
Medium
High
Temperature Accuracy
Low
High
Discontinuity at gain changes
—
±0.3°C
Firmware Size
Low
Medium
0
1
2
3
4
5
6
7
8
9
10
11
12
-50
-25
0
25
50
75
100
125
150
Thermistor Temperature (°C)
1
st Design
2
nd Design
-4
-3
-2
-1
0
1
2
3
4
-50
-25
0
25
50
75
100
125
150
Thermistor Temperature (°C)
Design # 1
Thermistor Emulator, Rvar



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